Position detection systems which, for example in the medical field, support a navigation of instruments, for example surgical instruments, are known per se. Such position detection systems may be optical, ultrasound-based or electromagnetic position detection systems. Thus, for example, electromagnetic position detection systems are known, in which a field generator generates an alternating electromagnetic field and provision is made for position sensors comprising coils. Currents are induced into the coils by the alternating electromagnetic field from the generator, wherein said currents depend on the alignment of a respective coil in relation to the alternating electromagnetic field. If a movable instrument is aligned with such position sensors in the form of sensor coils, it is possible to determine place and location of the instrument relative to a reference sensor which, for example, may likewise comprise coils. In so doing, the reference sensor as a patient localizer is preferably fixedly connected to a body part of a patient for else a different object).
For the purposes of navigating in body parts of a patient, the position of an instrument is typically detected by means of such a position detection system and the position of the instrument is displayed in slice images of the body part obtained by tomography. In order for this to work, the position values supplied by the position sensor of the instrument must be transferred into coordinates of the tomographic image of the patient. By way of example, to this end, the practice of generating a topographic image of the surface of a body part from a tomographic image of a patient is known, in order to correlate points on the surface of the topographic image (also referred to as model surface below) with those points on the surface of the real body part which are respectively contacted by a pointer instrument or sensing instrument. Thus, a transformation prescription for transforming position values detected by means of the position detection system into model coordinates may be produced within the scope of a registration method. To this end, a plurality of points are sensed on the real surface of the body part and the associated position values, which of course represent points on a real surface, are correlated with points on the model surface while maintaining their relative position with respect to one another in such a way that this results in the smallest possible error. A transformation prescription specifying how detected position values are to be converted into coordinates of the topographic image—also referred to as topographic model here—and therefore also into coordinates of the tomographic image or model, emerges herefrom.
The patient registration refers to the establishment of a transformation function for bringing position data detected during the operation into correspondence with position information in image data obtained prior to surgery, for example by tomography. By way of example, for the purposes of patient registration, as described above, a patient model is detected and a transformation function is established which, within the scope of the registration method, brings detected position data and a patient model resulting therefrom into correspondence with position information relating to the image data obtained prior to surgery.
For the purposes of determining the transformation function, the same geometric features in the model and in the image data (for example obtained by tomography) are established in the respective coordinate system. The two coordinate systems are then correlated by means of these features. The surface registration by means of a pointer instrument is generally widespread. Here, the skin surface of the patient serves as corresponding feature. The skin surface is sensed by means of a pointer instrument during surgery and brought into correspondence with the extracted skin surface from the image data.